Gas turbines have recently been increasingly combined with steam turbines to form combined gas/steam turbine power stations, so called combined power stations. In these power stations, the exhaust gases of the gas turbine circuit supply the energy for the downstream steam turbine circuit.
For this purpose, heat exchangers are arranged in the waste-heat boiler of the gas turbine and these heat exchangers extract the major part of the heat available in them from the exhaust gases of the gas turbine before the latter are delivered to the atmosphere through a chimney. Using this heat, water which has been supplied is evaporated in a multi-stage water/steam circuit and live steam is obtained. The latter is supplied to the steam turbine and is admitted to its blading rows so that the thermal energy is converted into mechanical energy. The steam turbine is coupled to a generator which produces electrical energy.
A relatively high utilization of the fuel is possible in this way and relatively high efficiencies can be achieved.
However, even in such combined power stations, part of the available heat energy is not used and is delivered to the ambient air as waste heat. Such energy losses must, however, be minimized if the efficiency of the power station is to be increased.
A combined power station is known from EP 05 19 304 A1 in which the energy of the cooling air, which is heated in the compressor and is provided for the gas turbine, is not delivered as waste heat to the external air but is used for converting an additional liquid medium into steam. A separate, second cooling system provided with integrated heat exchangers has to be configured for this purpose. It initially carries cooling water which is converted into steam when flowing through the heat exchangers and the steam is finally fed into the combustion chamber of the gas turbine. The separate cooling system can also feed the steam into the water/steam circuit of the waste-heat boiler, for which purpose the two are connected together.
Such a separate cooling system is found to be problematic where the combined cycle installation, i.e. the joint operation of the gas turbine and the steam turbine, has to be converted to the simple cycle, i.e. to the operation of the gas turbine alone. In this case, the exhaust gases from the gas turbine are no longer fed into the heat exchangers of the waste-heat boiler but are fed directly to the chimney. The water/steam circuit taking place in the waste-heat boiler is therefore made inactive and the supply of cooling water is stopped. So that the additional liquid medium, or the steam, of the second cooling system can then no longer penetrate into the heat exchangers, their connection must be interrupted and the connection to the combustion chamber or another consumer unit opened.
This requires complicated, mutually matched switching procedures and reduces the reliability of the system.
In order to arrange such a separate cooling system, a series of additional components, such as pumps, pipes and heat exchangers is necessary and these make the installation not only more complicated but also more expensive.
In the case of a leakage in the combined air/water cooling system, furthermore, the danger exists that cooling water may penetrate into the gas turbine and there lead to damage. At the very least, however, the effect of the openings in the rotor blades provided for the purpose of air cooling can be impaired by the impurities and minerals contained in the cooling water and this greatly endangers the functional capability of the gas turbine.